A commonly used approach for sharing a channel in unlicensed frequency bands (such as the 2.4 GHz frequency band used for wireless communication according to the IEEE 802.11 WLAN standard family) is based on carrier sense multiple access with collision avoidance (CSMA/CA). Effectively, a device that intends to make use of the wireless medium for transmission senses the channel and determines whether the channel is busy (in the following also: “in use”, “used” or “occupied”) or idle (in the following also: “not in use”, “unused” or “unoccupied”). If the channel is determined to be busy, the transmission is deferred whereas if the channel is determined to be idle a transmission is initiated. Just as the name “CSMA/CA” suggests, the idea is to avoid collisions by only initiating a transmission when the channel is not already used by another transmitting device.
In the IEEE 802.11 standard family (which is directed to Wireless Local Area Network, WLAN, communication), initiating a transmission usually requires generating a random back-off value reducing the risk that two devices that find the channel being idle start transmitting at the same time. In practice, other transmission coordination mechanisms may be used too. The details regarding how this initiation of a transmission is implemented are not part of the present disclosure since they are known to the person skilled in the art from the specifications of the respective IEEE 802.11 standard. Therefore, these details are herein not discussed further.
A critical component of the CSMA/CA protocol is how to determine whether the channel is busy (“in use”, “used” or “occupied”) or idle (“not in use”, “unused” or “unoccupied”). There are two fundamentally different ways to determine whether the desired channel is busy or idle. In the first approach, a receiver is searching for a specific (well-defined) signal or “preamble”. If found, the wireless medium (channel) is considered to be busy. Additionally, some implementations may consider the channel to be busy only if the signal is above a specific threshold value (a threshold power level). This approach is commonly referred to as signal detect or preamble detect (PD). IEEE 802.11 defines the PD threshold for its OFDM (Orthogonal frequency-division multiplexing) radio designs to be set to −82 dBm or less in the unlicensed 2.4 GHz and 5 GHz bands. That is, if a preamble of an IEEE 802.11 signal is detected at a power level of −82 dBm or higher, the channel must be classified as busy and a device must defer its transmission. On the contrary, if a device detects a well-known signal at a power level below the PD threshold level, the device may classify the channel as idle and may initiate a transmission. Often, however, an IEEE 802.11 STA (station, in the following also: “user device”) applies a PD threshold level lower than the required −82 dBm. Specifically, the PD threshold often coincides with the sensitivity threshold for the STA, which may be around −92 dBm. Basically, this means that the user device will defer to any successfully received transmission containing the well-known IEEE 802.11 preamble.
However, sometimes, the channel, in which a transmitter (e.g., a user device) intents to transmit, may be occupied by a signal that, e.g., is generated by a dissimilar system (e.g., not a WLAN device). In this case, it is not sufficient to only use PD in order to determine whether the channel is busy or idle. Therefore, in addition to PD, an IEEE 802.11 receiver sensing the channel also needs to consider the presence of other signals. This is done by detecting the energy level of any signal in the channel. In contrast to PD, this detection is performed independently of the actual type of signal or known preambles. The channel is declared as busy if the energy level exceeds a predefined threshold level and the channel is considered to be unoccupied (idle) otherwise. This way of determining the state of the channel is commonly referred to as energy detect (ED). In IEEE 802.11, the threshold level used for ED is −62 dBm.
As is readily understood, the lower the level that is used for declaring the channel as idle, the less “aggressive” the user device is in accessing the channel. So, comparing the levels for PD and ED, it can be concluded that an IEEE 802.11 system is relatively “nice” to other IEEE 802.11 systems in that it will not initiate a transmission if another IEEE 802.11 transmission at or exceeding −92 dBm (in practice) is detected, whereas if the transmission is caused by another system, the IEEE 802.11 system will instead consider the ED threshold and defer from transmission if the observed energy level exceeds −62 dBm.
To see what this means in terms of range, one may consider some reasonable values for an IEEE 802.11 system. A reasonable transmission (TX) power is 15 dBm. Furthermore, if the system is operated at 2.4 GHz a reasonable model for the propagation loss (PL) in dB is:PL=40+35 log10(d),  (1)
where the first term of 40 corresponds to the attenuation at a distance of 1 m (d=1) and the distance d is given in meters. For another carrier frequency than 2.4 GHz the constant will take another value.
With the typical PD and ED thresholds above, i.e., −92 dBm and −62 dBm, respectively, the corresponding PLs become 107 dB and 77 dB, respectively, for a transmit power of 15 dBm. Finally, using the above formula (1) for PL, it can be readily seen that this corresponds to distances of 82 m and 11 m, respectively.
At the same time, the required signal-to-noise-ration (SNR) for an IEEE 802.11 system using the most robust modulation and coding scheme (MCS) may be around 2 dB, which corresponds to a PL of 107 dB and a range of 82 m at 15 dBm transmit power. In fact, a PD threshold of −92 dBm is representative of the sensitivity level for the lowest (most robust, slowest transmission speed) MCS today's radios are commonly capable of. The number is found as follows. The thermal receiver noise power at room temperature in a 1 MHz bandwidth is −114 dBm, which can be found in books on communication theory and using reasonable assumptions. With a 20 MHz bandwidth, the noise power increases by 13 dB. Finally, we assume a radio implementation specific noise figure in the receiver of 7 dB, leading to a noise floor of −114 “dBm”+13 “dB”+7 “dB”=−94 “dBm”. With a required SNR of 2 dB, the sensitivity level of −92 dBm is obtained.
FIG. 1 provides an illustration of the situation described above. The station STA1 1, which is assumed to be located in the center of the small circle 3, would only detect non-Wi-Fi transmissions using ED if the corresponding transmitters would be located within the small circle 3. As calculated above, the small circle 3 is assumed to have a radius of 11 m for the present exemplary consideration. Further, a “coverage area” for an access point (AP) 5 has been calculated above to correspond to 82 m (assuming a sensitivity level of a receiver of −92 dBm). This coverage area is indicated by the large circle 7.
Considering that the access point (AP) 5 is not located within the small circle 3 and that the area of the small circle 3 represents (11/82){circumflex over ( )}2≈18/1000 of the coverage area of the AP 5 it becomes obvious that using ED for detecting WLAN transmission is insufficient. If instead PD is employed, the AP 5 operates within detection range, since it is located within the large circle 9 indicating a PD detection range of STA1 1.
Only STAs located in the opposite part of the AP's 5 coverage area cannot be detected under the described PD threshold.
The fact that the channel erroneously might be declared as idle when it in fact is busy is commonly referred to as the hidden node problem due to the fact that the transmitter, which is not heard, is hidden from the STA (STA1) performing the clear channel assessment (CCA). Hidden STA scenarios are susceptible to colliding transmissions. A Request-To-Send/Clear-To-Send (RTS/CTS) message exchange is commonly used counteracting this scenario. When a STA has data to send it senses the channel, and if found idle it transmits an RTS message to the intended receiver.
If the intended receiver successfully receives the RTS message it responds with a CTS message indicating a period of time to devices in its surroundings that it requests other devices to defer from medium access attempts.
However, cases may occur, in which a wireless communication system (e.g., a user device) is unable to perform preamble detection PD but can perform energy detection ED. In such a case, the choice of ED power threshold level, below which the wireless medium may be treated as idle, becomes the key parameter for determining performance. In known devices, in IEEE 802.11, the energy detection threshold is set to −62 dBm, as discussed above. As illustrated above, this value is too high for operating with low collision probability. In case a new (additional) IEEE 802.11 operating mode would not be able to rely on PD but solely operate under energy detection rules, radio performance would be detrimentally affected, which is a problem of prior art techniques.
In view of the above, known techniques and communication standards do not sufficiently deal with the aforementioned situation that a wireless device only relies on ED for performing a clear channel assessment.